Cracking furnaces are used in a two-stage cooling system in some plants for producing ethylene. A vertically arranged double-tube heat exchanger is usually provided in this case as a primary quench cooler and a conventional, vertically or horizontally arranged tube bundle heat exchanger as a secondary quench cooler.
Such a tube bundle heat exchanger is used as a process gas waste heat boiler for rapidly cooling reaction gases from cracking furnaces or chemical plant reactors while at the same time generating high-pressure steam as the cooling medium removing the generated heat.
A tube bundle heat exchanger is known from EP 0 417 428 B1, in which heat exchanger at least one tube bundle is enclosed by a casing, forming an interior space, which is formed between two tube sheets arranged at spaced locations from one another, wherein tubes of the tube bundles are held each on both sides in the tube sheets. The tube sheet is provided on the gas inlet side with open turn-outs concentrically enclosing the tubes and parallel cooling channels, which are in connection with one another and through which a cooling medium flows.
Further, a tube bundle heat exchanger is known from WO 01/48434 A1, which heat exchanger has a casing, which is under pressure, and a lower tube plate, which separates the interior space of the casing from an inlet distributor for the entry of the fluid to be cooled. The lower tube plate has passages for the fluid, and cleaning passages are arranged laterally close to the inner surface of the tube plate for connection to the outside of the casing, and said cleaning passages are intended for inserting a device through the casing in order to clean the tube plate at the foot of the tube bundle. Inspection passages may also be present close to the plate surface for a visual inspection of the zone to be cleaned.
High velocity of the flow of water over the tube sheet is very decisive in case of vertically arranged secondary quench coolers, in which the tube sheet at the gas inlet according or at the gas outlet represents the lowest point in the water system, in order to avoid harmful effects in respect to the tube sheet. Such effects arise, e.g., due to deposits as a consequence of corrosion and due to overheating as a consequence of the settling of solid particles on the tube sheet.
Small solid particles very frequently enter the water of the water flow arrangement of the quench cooler, especially during the start-up of such a plant, for example, for ethylene production. In addition, the water-side metal surfaces of the tube sheet, of the tubes and of the casing produce a layer of magnetite or Fe3O4. The magnetite layer protects the steel of the tube sheet and it always slowly regenerates itself from the metal surface at operating temperature, while a small quantity of particles consisting of magnetite is released into the water.
Besides the high velocity of the water flow, it is just as important to guide the water flow over the tube sheet away from sensitive areas of the tube sheet, e.g., the middle of the tube sheet with the highest heat flux, to areas in which effective blow-down can be employed.
The tube sheet of the secondary quench cooler is designed as a so-called membrane design and comprises a thin plate with a thickness of about 25 mm. The bundle tubes of the quench cooler are welded onto the thin plate.
No devices are provided on the plate for routing the water flow over the tube sheet of the gas inlet or of the gas outlet.